The invention relates to a method for pinpointing temporary interruptions in links, such as communication links, for transmitting digital signals, a device for implementing the method, and a system and communications network in which the method is used.
More in particular, the invention relates to a method for pinpointing a temporary interruption in a link between a first station and a second station on either side of the link, based on the points in time at which signal changes on the link reach the stations. In particular, the distance of said stations to the point where the interruption has occurred is determined in this manner. Below, the term "interruption", as used in this disclosur, designates the cause of the signal changes, even if no complete interruption is involved here--i.e., no complete loss of a signal transmitted over the link--or if the interruption is only momentary.
Many of the (temporary) interruptions in communication links arise during maintenance or management activities. There may be thought of, e.g., a temporary rerouting of a link, or of a (momentary) withdrawal of a printed-circuit board in an amplifier station. For attributing the responsibility for the failure caused in the information traffic by the interruption, it is of importance to determine where the interruption took place.
Admittedly, in the Japanese patent application JP-A-4-37321 [ref. 1] there is disclosed a method to determine the location of an interruption on the basis of the propagation delay of signal changes. In the event of said known method, however, a signal change specifically consists of the (substantial) loss of a carrier wave or information signal on the link, and said loss will generally be the direct result of an interruption, or at any rate a failure, in the link. The method is based on determining the difference in the time of occurrence of a change in the analogue signal value, determined on either side of the link. Use is made of the fact that on the analogue transmission path in question there is a signal which in the event of an interruption occurring in said transmission path is completely or substantially lost. The point in time of such loss of a signal cannot be determined exactly with simple means, although such should in fact be done on either side of the link in order to be capable of determining the difference with sufficient exactness. When digital signals are transmitted by way of the transmission path, special provisions should be installed to derive a signal from the digital signals which, in the event of an interruption in the link, undergoes such a characteristic change as in the known method. In addition, in the event of the known irethod, measured values, namely, times of occurrence of changes in the signal value between stations on either side of the link, must be exchanged and compared to one another. This is possible only by way of another link between the stations existing at that moment, or after restoration of the link on the basis of relatively inaccurate measured values determined earlier. Moreover, for determining the location of the interruption, the overall length of the link, designated by L in JP-A-4-37321, must be known.